Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CA 02369217 2002-O1-23
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Lamp ballast with SEPIC corwerter
Technical field
The invention is based on an electronic ballast
according to the preamble of claim 1. In particular, it
concerns the control design for an input stage of the
electronic ballast.
to Prior Art
The document~W096/03017 (Krummel) discloses an
electronic balhast for the operation of lamps,vwhich
has a two-stage structure. An input stage is operated
from the rectified mains voltage and provides a DC
1s voltage. This DC voltage is used to operate an
inverter., which generates a high-frequency (compared
with the frequency of the mains voltage) AC voltage for
the operation of lamps. The .input stage has the
following tasks: For reducing mains current harmonics
2o to regulation (IEC 1000-3-2) values, it has to correct
the power factor on the mains side. It also has to
provide a DC voltage which is substantially independent
-- of mains voltage fluctuations.
In said document, the input stage is designed
25 as a step-up converter. This very widespread solution
has the. advantage that it makes do with few components
and achieves quite high efficiencies. However, the
input stage with a step-up converter has the following
disadvantages: The DC voltage which is delivered is.
3o always greater than. the maximum of the mains voltage;
it generates strong RF interference the starting,
current is high and the control needs a converter which
either functions in analog mode or, if it functions in
digital mode, has a sampling rate which is
35 substantially higher than the mains frequency. The last
disadvantage has the following background: The step-up
converter is a switched-mode converter. That is to say,
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it has a switch whose duty ratio significantly affects
the input current. The term "duty ratio" generall y
denotes the ratio of the time during which the switch
is on to the time during which the switch is off. In' '
order to achieve a high power factor, the duty ratio
needs' to be adjusted to the mains voltage.
Less widespread is the use of a so-called SEPIC
converter in the input stage. It is described in the
following document: '
to R. Antonio et al.: "EVALUATION OF BOOST, SEPIC
AND CUK TOPOLOGIES AS POWER FACTOR.CORRECTION STAGE IN
ELECTRONIC BALLAST APPLICATIONS", IEEE Power
Electronics Congress CIEP, Mexico,.1994. Although the
SEPIC converter requires more components, the
aforementioned disadvantages of the step-up converter
can be avoided. In this case, themode with a .
'discontinuous current profile described in said
document plays an important role: In this mode it is .
possible., with a constant duty . ratio for' the 'Switches . '
2o contained in the SEPIC converter, toachieve a power'
factor which leads to regulation mains current
harmonics.
Description of the Invention
It is an object of. the present invention to
2s provide an electronic ballast according to. the preamble
of claim l, which makes use of the advantages of the
SEPIC converter in an inexpensive way.
This.,abject is achieved, in the case of an
electronic ballast having the features of the preambles
30' of claims 1 and 3, by the features of the
characterizing parts of claims 1 and 3. A particularly
advantageous configuration is given in claim~2.
In order to control the output voltage and
adjust the power factor of the SEPIC converter, an
35 analog controller is used in said document. According
to the invention, a digital controller is used to
control the SEPIC converter. Further, the abvve-
explained conditions for controlling the step-up
converter which dominates the marketplace make it
.~ ~.
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obvious that, when a digital controller is used, it
must have a sampling procedure that can resolve the
mains frequency. In this context, the term "analog
controller" is intended to mean a controller which
reads in. and processes actual values, and outputs
control values, as a continuous function of time. A
digital controller functions as a discrete function of
time. The actual values are sampled only at discrete
points in time, between which there are sampling
1o intervals. A new control value is likewise output at
discrete points in time. This gives rise to control
intervals within which the control value is constant.
For the step-up converter, it has been found. that the
sampling arid control, intervals need to be shorter than
1s .~ 1~ of the period of the .mains frequency, in , order ' to .
meet the requirements of said applicable regulations.
The invention uses the aforementioned property
of . a SEPIC converter that, i~n the mode with a
discontinuous current ~rofile;~ a sufficiently..good
2a power factor can be achieved with ,a constant duty
ratio. The duty ratio is not adjusted to the
instantaneous mains voltage, but instead merely the
output voltage of the SEPIC converter is controlled.
The output voltage need not be sampled at intervals
25 which are shorter than 1~ of the period of the. mains
frequency. According to the invention, the control .
intervals', i.e. the time during which the duty ratio is
kept constant, are now at least as long as 1°s of the
period of.the mains-frequency. For.a mains frequency of
30 50 Hz, for example, a control interval that is at least
0.2.ms long is,hence obtained.
A commercially available microcontroller
constitutes a flexible solution far implementing the
controller according to the invention for the input
s5 stage of the electronic ballast. Since the invention
requires only lorig sampling intervals (compared with
the control of a step-up converter), an inexpensive
microcontroller can be selected.
CA 02369217 2002-O1-23
Description of the drawing
The invention will be explained in more detail
with reference to an exemplary embodiment.
Capacitors are denoted below by letters C,
inductors by L; diodes by~D and-switches by S, in each
case followed by a number. w
' The figure shows an electronic ballast with a~
lamp. attached and an input' stage according to the
invention with control by a microcontroller.
- 1o The electronic ballast is. supplied by a~
. rectified mains voltage V1, which is joined .by its
- negative terminal to the ground potential M. ~'he series
circuit~consisting of an inductor~Ll and a switch.S1 is .
. attached to the positive terminal of the rectified
mains voltage V1. S1 is connected to the ground
- potential M. It is preferably designed as a
semiconductor switch. The series.circuit consisting of
,.-. a capacitor C1 and an inductor L2 is attached to the
connection - point , of 'L1 and S1 . 'L2 . is connected to the
2o ground potential M. L1 and~L2 may be coupled. This
reduces .the RF. interference which is produced on the
' mains feed. The series circuit consisting of a diode D1
and a capacitor C2 is attached t.a the connection point
of C1 and L2. C2 is connected to the ground potential
M. The boxed area P constitutes the input stage. of the
electronic ballast. It contains the components L1, L2,~
C1, C2, S1 and D1, which form a SEPIC converter: At C2,
the SEPIC converter makes a DC voltage available, with
.which the inverter. W can be..supplied. The lamp Lp is
3o attached to the output of the inverter W. The inverter-
W is preferably designed as a half-bridge inverter. It
contains the coupling circuit for the lamp Lp..
In order to control the input stage P, a
microcontroller UC is provided. It samples the output
voltage of the input stage P from the potential DC and
sends, to the control electrode G1 of the switch 51, a
signal with a duty ratio that leads to a desired
setpoint value for the DC voltage at the potential DC.
According to the invention, the potential DC is sampled
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by the microcontroller at sampling intervals which are
at least as long as l~ of ,the period of the mains
frequency. Therefore, the control intervals are also at
least as long as 1~ of the period of the mains
s frequency..,
Between the~microcontroller LFC .end the inverter
W, there is the connecting line B. The microcontroller
UC can hence also undertake control and regulating
.tasks for the invexter w..This is possible even with
la inexpensive microcontrollers UC, ~ primarily since, .
. according to the invention, the microcontroller UC
needs to perform regulating tasks for the input stage P
only at long sampling intervals.
The exemplary embodiment is equipped with only
15 one lamp. The invention. can, however, also be applied
toapplications with a plurality of lamps. '
